Photodegradable compositions are useful, for example, for making agricultural mulching film which can be used to protect seeds and seedlings but which rapidly degrades so that it does not interfere with ripening, harvesting or replanting. Such compositions are also useful to prevent the accumulation of litter.
It is well known that polymeric materials, especially plastics, are degraded by the action of heat, light and air. Apart from polymers specifically designed for high temperature applications, most polymers undergo heat degradation only during melt processing and forming. Usually, however, polymers are subject to light and air throughout their life, and the length of their life can be controlled by a suitable choice of additives.
Various systems have been suggested for controlling the life of polymers. For example, polymers have limited life if carbonyl groups are incorporated into the polymer by copolymerisation or graft copolymerisation. This technique has the disadvantage that separate polymerisations have to be carried out to produce polymers with different lifetimes. Consequently, it is often preferable to add pro-degradants to a polymer during processing.
Pro-degradants or pro-oxidants are usually compounds of multi-valent metals. U.S. Pat. No. 3,454,510 discloses as pro-oxidants the acetylacetonates of Mn, Co, Cr, Fe, Cu or V, the C.sub.8-18 alkyl acetoacetate or C.sub.8-18 alkylbenzoylacetate complexes of the same metals, Zn or Ni, or the stearates or oleates of Mn(II) and Co(II). Addition of one or more of these compounds to a polymer enables films to be prepared which last only during the growing period of a crop.
U.S. Pat. No. 4,048,410 discloses the addition to polymers of organic chelating agents which are expected to extract suitable metal ions from the environment and thus be subject to accelerated degradation. Many chelating agents are mentioned, including acetylacetone, acetonyl-acetone, benzoylacetone, thenoyl trifluoroacetone, trifluoroacetylacetone, dibenzoylmethane, dipivaloyl methane, n-dodecylacetylacetone, n-dodecylbenzoylacetone and diethyldithiocarbamate salts.
U.S. Pat. No. 4,121,025 discloses systems in which degradation-promoting metals are complexed with stabilising organic compounds or groups. Examples of such metals are Fe, Ag, Pd, Mo, Cr, W and Ce. Examples of the compounds or groups include tetramethylthiuram disulphide and bis-dithiocarbamates.
GB-A-1356107 discloses compositions containing Fe, Mn, Ag, Pd, Mo, Cr, W or Ce with a complexing agent which itself or as a metal complex is an antioxidant. Dialkyldithiocarbamic acids, iron dialkyldithiocarbamates and the specific combination of ion dibutyldithiocarbamate and zinc dibutyldithiocarbamate are described.
GB-A-1586344 discloses compositions containing at least one photo-activating metal complex (the complexing agent being stabilising) and at least one light-stabilising complex of Ni, Co or Cu. Examples are iron dimethyldithiocarbamate with nickeldibutyldithio-carbamate and iron dibutyldithiocarbamate with cobalt dibutyldithiocarbamate.
GB-A-2187193 discloses compositions containing transition metal complexes of oxygen ligands and transition or Group II or IV metal complexes of sulphur ligands, wherein the second complex is less than one-third of the total complex. Examples are iron acetylacetonate, iron acetylacetonate with zinc diethyldithiocarbamate, and iron acetylacetonate with nickel diethyldithiocarbamate. The sulphur complex is for thermal stabilisation.
EP-A-0216412 discloses mixtures of a copper ketonate or diketonate, e.g. copper acetylacetonate, and a zinc dithiocarbamate, e.g. zinc dimethyldithiocarbamate.
The known systems are effective but have disadvantages. For example, the very powerful iron acetylacetonate requires a stabiliser, e.g. zinc diethyldithiocarbamate, not only to provide an induction period but also to prevent deterioration of the plastic during initial processing (see FIG. 1 of GB-A-2187193).